1. Field of the Invention
The present invention relates, generally, to an automatic transmission and, more specifically, to an automatic transmission designed for use with a hybrid vehicle.
2. Description of the Related Art
Generally speaking, land vehicles require a powertrain consisting of three basic components. These components include a power plant (such as an internal combustion engine), a power transmission, and wheels. The power transmission component is typically referred to simply as the “transmission.” Engine torque and speed are converted in the transmission in accordance with the tractive-power demand of the vehicle.
Hybrid vehicles employ two separate power plants. One power plant is typically an internal combustion engine. The other is an electric motor. The electric motor is powered by electrical energy stored in a bank of batteries that are typically also supported on the vehicle. The batteries may be recharged by the internal combustion engine via an electrical generator that is driven by the internal combustion engine or from another external power source.
Controlling and regulating transmissions to achieve the desired vehicle occupant comfort goals in an efficient and cost effective manner is a complicated matter. There are a large number of events to properly time and execute within the transmission for each shift to occur smoothly and efficiently. Furthermore, since the control of automatic transmissions is carried out by hydraulically actuating the various components within the transmission, it is important to provide a stable hydraulic pressure. Since hydraulically actuated devices respond in a predetermined and a precise manner for the given pressure supplied to actuate them, inaccurate control of the hydraulic pressure causes inaccurate operation and control of the transmission. Establishing and maintaining a stable hydraulic pressure in an automatic transmission can be problematic. As previously mentioned, a pump is employed to provide pressurized hydraulic fluid for the control and actuation of the transmission. In addition, the clutches and gear assemblies are lubricated and cooled by a secondary flow of hydraulic fluid. With conventional vehicles, the pump is mechanically driven by a power take-off from the engine. Thus, the hydraulic pressure delivered from the pump increases as the pump speed increases in response to an increase in engine speed.
The challenges of maintaining hydraulic pressure delivered from the pump in a conventional vehicle are only complicated in a hybrid vehicle. In a hybrid vehicle, the electric motor is operable when the vehicle is driven short distances at slower speeds, such as in stop-and-go traffic and city driving. However, even when the electric motor drives the vehicle, the clutches, as well as other components of the transmission, still need to have access to pressurized transmission fluid. Accordingly, it is known to employ a separate pump that is driven by the electric motor to ensure that the transmission has sufficient access to pressurized fluid when the electric motor is operable. Thus, transmissions used for hybrid vehicles typically employ two separate pumps that act independently of one another to provide pressurized fluid to the transmission under all operating conditions. This essentially requires two redundant pumps of similar size and capacity that operate alternatively depending on whether the internal combustion or the electric motor is driving the vehicle. This arrangement results in increased costs and complexity as the transmission must be configured to receive hydraulic fluid from two separate sources depending on which portion of the hybrid's power plant is operational. In addition, there is a certain inefficiency in having two pumps that serve the same purpose but are alternatingly operable depending on the source of motive power used to drive the hybrid engine.
One type of transmission known in the art has two clutches and is generally referred to simply as dual, or twin, clutch transmissions (DCTs). The dual clutch structure is most often coaxially and cooperatively configured to derive power input from a flywheel arrangement. However, some designs have a dual clutch assembly that is coaxial, but with the clutches located on opposite sides of the transmissions body and having different input sources. Regardless, dual clutch transmissions typically include one power transmission assembly on each of two input shafts concomitantly driving one output shaft. Each clutch and associated gear sets can be shifted and clutched independently. In this manner, uninterrupted power upshifting and downshifting between gears, along with the high mechanical efficiency of a manual transmission is available in an automatic transmission form. Thus, significant increases in fuel economy and vehicle performance may be achieved through the effective use of certain dual clutch transmissions. These factors make the dual clutch transmissions an attractive component for a hybrid engine.
While the automatic transmissions known in the related art have generally worked for their intended purposes, there remains a need in the art for a transmission for a hybrid engine having improved operational interaction between the power plants of a hybrid engine and the transmission. In addition, there remains a need in the art for such a transmission used in connection with hybrid engines that are less complex and costly to manufacture than current systems available in the market. At the same time, there remains a need in the art for a transmission designed for use with a hybrid engine that is capable of quickly and efficiently providing pressurized hydraulic fluid for the control and actuation of the transmission as well as for cooling the various components of the transmission in a cost-effective manner.